Typically, making accurate relative frequency measurements between two distant radio receiving systems may be based on two main strategies. The first strategy is to provide both receivers with very accurate timebase frequency references, such as Rubidium-based atomic clocks. The difference of two such accurate frequency references will be very small, resulting in a negligible error. It is common practice to discipline such atomic clocks using signals received from the Global Positioning System (GPS). Given the short term inaccuracies of GPS, and its low signal strength, the integration time required to obtain a high accuracy frequency (minutes to days) may be so long that only the highest quality oscillators will not have drifted significantly during the integration period.
The second strategy typically involves deploying cooperative reference transmitter devices into the environment. The transmitter devices may be deployed at known positions, which may be distinct from either of the two signal collectors. Ideally, the transmitter devices may be deployed relatively close to a target transmitter that is to be located. Comparing the frequency differences obtained by processing the reference transmitter's waveform through a geolocation algorithm versus that predicted a priori from the Doppler shifts shat should be obtained given the radio carrier frequencies and kinematics of the collectors and the reference transmitter may allow the error in relative frequency to be inferred.